SHOULDER GIRDLE AND ANTERIOR LIMB 515 
was made just posterior to an imaginary line passing dorso- 
ventrally midway through the embryo. The ectoderm and meso- 
derm were removed. The wound, after being cleaned from all 
free mesoderm cells, was then covered over by a circular piece 
of ectoderm, cut to fit the excision, taken from an embryo which 
had been properly stained in an aqueous solution of Nile blue 
sulphate. In about one-half hour this stained dise of ectoderm 
had healed in and its size and position was then indicated by a 
camera drawing (text fig.2). The heavily stippled area represents 
2 3 
Fig. 2. Camera drawing of an embryo (E. Tr. Ext. 12) in the stage of high 
medullary folds. The heavily stippled area represents a circular piece of ecto- 
derm taken from an embryo previously stained in a solution of Nile blue sul- 
phate. This occupies the wound resulting from the extirpation of the limb 
mesoderm. X 10. 
Fig. 3 Camera drawing of the same embryo (E. Tr. Ext. 12) shown in figure 
2 thirty hours later, showing the position of the inserted stained dise of ectoderm 
with reference to the somites. X 10. 
the circular piece of transplanted blue ectoderm, which is in 
marked contrast to the brownish-yellow ectoderm of the host. 
The position of this inserted piece of ectoderm, when develop- 
ment had proceeded so that the somites had become visible, 
served to indicate whether or not the limb rudiment had been 
removed. If the stained dise eventually came to lie just ventral 
to the third, fourth, and fifth somites (text fig. 3), then it could 
be said with more or less certainty that the limb rudiments had 
been removed. When the extirpated area was taken from a more 
posterior position than that shown in text figure 2, the stained 
disc which covered the wound came to lie ventral to the fourth, 
fifth, and sixth or the fifth, sixth, and seventh somites. This 
indicated that only a portion of the limb rudiment had been 
removed and regeneration could be predicted. 
